Fundamentals; Chlorine Dioxide For Water Treatment; Functional Sequence - Grundfos Oxiperm OCD-164 Montage- Und Betriebsanleitung

3. Fundamentals

3.1 Chlorine dioxide for water treatment

Properties of chlorine dioxide
• Strong and fast oxidation and disinfection agent.
• Applications in the treatment of drinking, service, cooling, and
waste water.
• Chemically unstable compound
– Can explosively decompose into chlorine and oxygen when
heated.
– Must be generated on site as required, since storage in
cylinders is not possible.
Advantages of chlorine dioxide compared to chlorine
• Largely good to very good bactericidal, virucidal and sporicidal
effects in the complete pH range of drinking water (pH 6.5 - 9).
The disinfecting effect of chlorine decreases with increasing
pH value.
• No or reduced forming of trihalogen methanes.
• No generation of chloramines with ammonium or amino
compounds.
• Highly reduced potential for generation of organic halogen
compounds of high molecular weight.
• Good stability in water. Long bactericidal and bacteriostatic
protection in the water network.
3.1.1 Preparation of chlorine dioxide
The chlorine dioxide preparation system was specially developed
for the continuous or discontinuous preparation of a chlorine
dioxide solution for water disinfection. The chlorine dioxide is
generated according to the hydrochloric acid/sodium chlorite
procedure in line with the following stoichiometric equation:
5 NaClO + 4 HCl <=> 4 ClO
2
Sodium chlorite + Hydrochloric acid <=> Chlorine dioxide +
Sodium chlorite + Water
Warning
The system must only be operated using a 9 %
hydrochloric acid solution and a 7.5 % sodium
chlorite solution.
Commercially available solutions such as 24.5 %
sodium chlorite or 32 % hydrochloric acid would
generate an explosive concentration, and must
therefore never be used undiluted in the system.
This system uses a 7.5 % NaClO
solution in a volume ratio 1 : 1 for chemical reaction.
The reaction time is approx. 10 minutes. This application uses a
multiple stoichiometric excess of hydrochloric acid for the
following reasons:
• A non-critical chlorine dioxide concentration of 20 g ClO
generated in the reactor.
Warning
Gaseous chlorine dioxide is explosive above a
concentration of 300 g/m
• A good yield of chlorine dioxide is achieved with excess acid
of 250-300 %. A further increase in the excess acid only
results in a small improvement in the efficiency.
• Excess acid shifts the equilibrium of the disproportionation
reaction between hydrochloric acid and sodium chlorite to the
right, resulting in an optimum yield.
10
+ 5 NaCl + 2 H O
2 2
solution and a 9 % HCl
2
/l is
2
3
.

3.2 Functional sequence

• Three components are required to generate a chlorine dioxide
solution:
– Hydrochloric acid (HCl)
– Sodium chlorite (NaClO )
2
– Dilution water (bypass).
The added quantities of these components are defined by the
process, and must not be changed. The flows of the individual
components are therefore monitored by flowmeters and flow
controllers.
Hydrochloric acid (9 % solution) and sodium chlorite (7.5 %
solution) are dosed into the reactor with a volume ratio 1 : 1.
There they react together, and generate an uncritical chlorine
dioxide concentration of 20 g/l.
Following the reactor, the chlorine dioxide solution is diluted by
the bypass water into a solution ready for use.
ClO solution
2
Reactor
NaClO
2
Bypass water
Fig. 2 Preparation of chlorine dioxide solution
HCl